| The middle and late age of Middle Jurassic is a period with large-scale paleoclimate change and ancient plant types change around the world.Influenced by the extensional depression and early stage of Yanshan movement,a set of superimposed or interbedded organic rich sediments,Dameigou formation,was formed at the northern margin of Qaidam Basin.These lacustrine-swamp organic rich sediments consist of four types of lithologies including coal,carbonaceous mudstone,oil shale and dark mudstone.The study of organic rich sediments in lacustrine-swamp facies has great scientific value not only in determining the geochemical records of paleoclimate,paleobotany and paleoenvironmental changes,but also in reconstruction of the coupling relationship among environment,biology and sedimentary organic matter in geological period.In practice,the study is important to determine the characteristics,development and distribution of different types of source rocks,and to predict hydrocarbon generating potential in lacustrine-swamp facies.Lasted for four years,the Dameigou formation of Middle Jurassic in Chaiye1 well,as the research target of this thesis,has been conducted for well log interpretation,sequence stratigraphic analysis,and more than 1430 tests which include XRD,ICP-MS,GC-MS,vitrinite reflectance and stable carbon isotope on 122 core samples.The methodology in this thesis is a multi-disciplinary approach including organic petrology,biomarker geochemistry,elemental geochemistry,sedimentology and petrophysics.Based on sequence stratigraphic framework,the sedimentary cycle of mineral composition,maceral composition and trace elements distribution in Dameigou formation are analyzed.According to the factors of paleoclimate,paleobotany and sedimentary environment,geochemical response to sedimentary cycle,paleoclimatepaleobotany change is constructed,and oil and gas potential of the source rocks in the Dameigou formation is further evaluated.The main achievements and innovative understanding are as follows:1.Sequence stratigraphy and petrological analysis show that the lacustrine-swamp sediments of the Dameigou formation has an incomplete cycle from delta plain,delta front,semi-deep lacustrine,deep lacustrine,semi-deep lacustrine to shallow lacustrine.2.Bounded by the deep lacustrine facies in the middle and upper part of the seventh member of the Dameigou formation,a humid-arid-humid paleoclimate cycle is indicated by the compositional characteristics and cyclic variation of minerals and trace elements in different microfacies.The oil shale was deposited in a transgressive cycle in arid deep-lacustrine facies.Meanwhile,the deep lacustrine,semi-deep lacustrine subfacies,and the swamp microfacies have relatively high paleo productivity.3.Maceral composition,OM source characteristics reveals the depositional environment evolution: the high-plant input decreases from delta plain to delta front,semi-deep lacustrine and deep lacustrine is controlled by sedimentary environment,paleoclimate and redox condition.The deep lacustrine subface dominated by sapropelinite and exinite groups,with high gamma wax content,presents dual biological source of aquatic algae and terrestrial higher plants in near-source environment with arid paleoclimate.The vertical variation of the relative content of cadalene,retene,and simonillite might be related to the paleoclimate cycle of the living condition and/or phytoecommunity of coniferous plants that constitute the main high plant source materials of middle Jurassic epoch4.Through the analysis of trace elements,biomarkers and organic petrology,the aquatic environment and cyclic change of different sedimentary microfacies are determined in Jurassic coal measures.From delta to deep lacustrine facies,the aquatic environment gradually changes from oxic to dyoxic and anoxic.The anoxic stratified aquatic environment exists in the microfacies of underwater distributary bay in delta front.The clear aquatic environment with inner circulation of lake water exists in the semi-deep lacustrine facies,and the deep lake facies is the brackish aquatic environment with strong stratification and reduction.5.By analyzing the frequency distribution characteristics of vitrinite reflectance and its relationship with lithological cycles and sedimentary environment,we have revealed the geological causes of anomalous vitrinite reflectance in the Jurassic coal measures in northwest China.It is pointed out that the transport distance of high plants(represented by maceral composition)and the sedimentary environment(represented by hydrogen index change)directly controls the enhancement or suppression of vitrinite reflectance.6.Based on the detailed geochemical records of lacustrine-swamp environment evolution in the Middle Jurassic epoch,the major factors affect the OM enrichment are specified and the genesis model of organic-rich sediments in the lacustrine-swamp environment is established.It is proposed that the OM source and the sedimentary environment jointly controls the abundance of OM(TOC)and the oil-prone or gas-prone tendency(maceral composition)in the lacustrine-swamp sediments.The major oil-prone source rocks of the middle Jurassic Dameigou formation were mainly deposited in deep lacustrine with hot and arid paleoclimate,peat-swamp microfacies and interdistributary bay with humid paleoclimate. |
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